The computer system has closed and open loop. Until you reach operating temperature appx. 192 degrees the system will run rich using more fuel its like running with the choke on in an older vehicle. You won't have as much control over timing.
Check base timing unplug the sprout and set to factory specifications. Shut down and plug in the sprout. Timing affects the operating temperature.
I had a man come in with a Chevy S-10 with the thermostat stuck open he claimed low mileage under 8mpg even with cheap fuel about 86 cents a gallon he was aware of it. A new thermostat was installed and it went back to normal appx.16mpg. With the throttle body you could see the flow. Since your buying the fuel its up to you. I was really looking for something else but the scanner said the computer was reading about 140 degrees coolant.
But an engine not timed correctly runs hotter.